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Research in the opioid field has relied heavily on the use of standard agonist ligands such as morphine, [d-Ala(2)-MePhe(4)-Glyol(5)]enkephalin (DAMGO), U69593, bremazocine, [d-Pen(2)d-Pen(5)]enkephalin (DPDPE), and deltorphin-II as tools for investigating the three major types of opioid receptors, MOP (μ), KOP (κ), and DOP (δ), that mediate antinociception. The functional selectivity of these ligands has been based on the assumption that opioid receptors exist as homomers. As numerous studies in cultured cells have suggested that opioid receptors can associate both as homomers and heteromers, we have investigated the selectivity of these standard ligands using intracellular calcium release and [(35)S]GTPγS assays in HEK-293 cells that contain singly and coexpressed opioid receptors. The present study reveals that morphine and DAMGO, traditionally classified as μ selective agonists, selectively activate μ-δ heteromeric opioid receptors with greater efficacy than homomeric opioid receptors. Moreover, standard ligands that have been widely employed as κ- and δ-selective agonists display little or no differences in the activation of homomeric and heteromeric opioid receptors. The far-reaching implications of these results are discussed.